Stan Lee's appearance as a World War II veteran in Avengers: Age of Ultron is a reference to his actual military career during the war.

Stan Lee is the only actor to appear in all Marvel Cinematic Universe movies, although all his appearances are cameos.

With the exception of Inhumans, he has also appeared in all MCU TV shows, although some of his appearances are in pictures (most notably all of his cameos in Netflix shows), rather than physical like all of his film cameos.

In Stan Lee's cameo in Avengers: Age of Ultron, after getting drunk, Stan Lee says "Excelsior", the same phrase with which he ended his weekly "Stan's Soapbox," which appeared in every Marvel comic book.

In Stan Lee's Guardians of the Galaxy cameo, when Rocket profiles him, "Excelsior!" can be seen written in the Skrull letters.